The present invention relates to a method and an apparatus for the reduction of the magnetic identity for ship parts which are movable for their bearing and elevation, primarily artillery guns, the aiming and possibly also firing of which are controlled by a computer. The reduction of the magnetic identity of the part is accomplished by means of an active, computer-controlled electromagnetic compensation.
The present invention will be exploited in the first instance on mine hunters and other marine vessels which are only armed with light and/or medium-heavy antiaircraft and general target guns.
If they are to succeed with their principal task, which, as the name implies, is to hunt mines, the mine hunters must not themselves trigger off the mines they are supposed to hunt. The magnetic identities of the mine hunters projected today must therefore be kept below the anticipated sensitivity of the magnetic mines of tomorrow.
The magnetic interference field around a metal body, which thus gives this method body its magnetic identity, is primarily dependent upon the magnetic properties of the metal, the iron length, for example the greatest coherent length of magnetic material in the body, the intensity of the terrestrial magnetic field at the place where the body is situated and the direction of the body in relation to the terrestrial magnetic field.
Individual, large solid iron masses on a ship, such as engines etc, can be magnetically compensated with the aid of magnetic coils built into the hull and deck through which accurately adapted electric currents can be continuously passed.
Electrical compensation of a complete steel ship, however, requires far too high units of power to afford a realistic solution to the problem. In view of this, normal practice has been to build the mine hunters in a non-magnetic material such as wood, aluminum or, in recent times, plastic. All large iron objects on board, such as engines, propeller shafts and heavy weapons, have subsequently been magnetically compensated individually. Since the magentic interference field of a body is dependent upon the terrestrial magnetic field which, in turn, is dependent upon latitude and also exposed to local variations which are not directly dependent upon latitude, the magnetic compensation of ship parts has had to be restricted with the technology available hitherto to the latitudes and areas in which the ship concerned has had its main duties. This has led to major problems for such marine vessels when they move over large areas of sea.
For ship guns with a minimized magnetic identity, this will be largely dependent upon their elevating systems, such as barrel and mechanism, since the mounting can normally be divided into several separate parts with non-magnetic material between the parts. As a guide value, it can be considered that if a coherent iron length gives rise to a certain magnetic interference field, then the same iron length, divided into two equally long partial lengths disposed one after the other but separated by a non-magnetic material only gives rise to an interference field which is 25 percent of the interference field of the coherent iron length.
If the guns are equipped with large platforms of electrically conducting material, so-called eddy currents are formed when the ship moves in the sea and thus through the terrestrial magnetic field. The alternating field then formed may, however, be relatively simply limited in that the branch circuits for the eddy currents are broken by non-electrically conducting material which divides the platforms into several parts.
With the technology available hitherto, it has only been possible to magnetically compensate the traversing and elevating systems of the ship's guns in a fixed zero position relative to the vessel. The implication, then, is that it has not been possible to bring the ship's gun into the firing position without jeopardizing the magnetic compensation of the ship.
The normal procedure hitherto, therefore, has been to restrict the armament of the mine hunters to only one or a few light antiaircraft guns. This in turn highly limits the utility of the ships for other combat tasks than pure mine hunting, and capability of the ships to undertake their own protection against enemy aircraft and small surface water vessels.